As mentioned, the HD 7750 is the best gaming card for the 300 watt power supply. HERE is a list to look at. It uses less power than the GT 640 mentioned and will work just fine. The ones I linked all have the faster GDDR5 memory vs the slower DDR3 type. Note that if you choose the low profile Sapphire card which was linked above, you will have to change brackets to the full size. The 660 mini tower takes a full size bracket.

I wanted to add that there is a really good deal right now on this MSI card HERE. It has DDR3 memory, but the price may be worth it. Only $49 after the MIR. Also note that on the DDR3 cards, the memory clock speeds are running at just 800mhz vs 1125mhz on the DDR5 cards. However, you are getting 2GB of video ram on this MSI card, but it does not make it faster than the 1GB DDR5 cards with the higher clock speeds.

As mentioned, the HD 7750 is the best gaming card for the 300 watt power supply. HERE is a list to look at. It uses less power than the GT 640 mentioned and will work just fine. The ones I linked all have the faster GDDR5 memory vs the slower DDR3 type. Note that if you choose the low profile Sapphire card which was linked above, you will have to change brackets to the full size. The 660 mini tower takes a full size bracket.

I wanted to add that there is a really good deal right now on this MSI card HERE. It has DDR3 memory, but the price may be worth it. Only $49 after the MIR. Also note that on the DDR3 cards, the memory clock speeds are running at just 800mhz vs 1125mhz on the DDR5 cards. However, you are getting 2GB of video ram on this MSI card, but it does not make it faster than the 1GB DDR5 cards with the higher clock speeds.

If you upgraded the power supply to say around a 650 watt unit, then you could get just about anything you want if you want to put this kind of money into it. Of course it has to physically fit and due to the hard drive cage location in the 660, you might be limited to a 10" long card. Still a lot of good cards that would work though.